Chapter 13:
Principles of Unit Protection

By Ben Ramesh

From Practical Power System Protection

13.1 Protective Relay Systems

The basic function of protection is to detect faults and to clear them as soon as possible. It is also important that in the process the minimum amount of equipment should be disconnected. The ability of the protection (i.e. relays and circuit breakers) to accomplish the latter requirement is referred to as 'selectivity'.

Speed and selectivity may be considered technically as figures of merit for a protection scheme. In general; however greater the speed and/or selectivity, the greater is the cost. Hence, the degree of speed or selectivity in any scheme is not purely a technical matter, it is also an economic one.

13.2 Main or Unit Protection

The graded overcurrent systems described earlier do not meet the protection requirements of a power system. As seen in Chapter 10, the grading is not possible to be achieved in long and thin networks and also it can be noticed that grading of settings may lead to longer tripping times closer to the sources, which are not always desired. These problems have given way to the concept of 'unit protection' where the circuits are divided into discrete sections without reference to the other sections.

Ideally, to realize complete selectivity of protection, the power system is divided into discrete zones. Each zone is provided with relays and circuit breakers to allow for the detection and isolation of its own internal faults.

This ideal selective zoning is illustrated in Figure 13.1. The protection used in this manner -essentially for internal...

Circuit tracers or signal tracers are used to troubleshoot circuits. They are relatively simple electrical testing instruments that enable electricians and other users to locate, trace and identify common electrical problems such as breakers, shorts, and open circuits.

Topics of Interest

14.1 Introduction
Two commercially available systems have been chosen as typical examples to illustrate the concepts. However there are other similar products on the market using essentially the same...

System downtime is reduced when overcurrent protective devices are selectively coordinated. Designers should strive to limit the outage to only that circuit which is overloaded or which has faulted in...